After Rafael's patch series 'sched/cpuidle: Idle loop rework' has been
merged in mainline kernel, it perfectly resolved the Powernightmares
issue [1] with not stopping the tick during the idle loop; we verified
this patch series on Arm platform (96boards Hikey620 with octa CA53
CPUs) with the rt-app [2] program to generate workloads: a single task
with below combinded configurations with period 5ms and duty cycle
1%/3%/5%/10%/20%/30%/40%.
After run these testing cases, we found the CPU cannot stay in deepest
idle state as expected, the issues essentialy are related with sched
tick:
The prominent issue is the criteria for decision stopping tick; now
the criteria is checking expected interval is less than TICK_USEC, but
this doesn't consider from the perspective of idle state parameters, so
we can observe the CPU even has enough sleeping time but it cannot run
into deepest idle state; this is very serious for some specific ducy
cycle cases.
Another issue is after tick keeping running in idle state, the tick
can heavily impact on 'menu' governor metrics, especially it will
introduce many noise for next event correction factors.
This patch series tries to fix these two issues; patch 0001 wants to
define a time point to distinguish for stopping or not, this time point
consideres the factors from tick period and the maximum target residency
and use prediction period to compare this time point to decide if need
to stop tick. Patch 0002 wants to always to give compensation for tick
event so that dimiss the tick impaction on correction factors for next
time prediction.
Blow table are comparison results for testing cases between without and
with this patch series; we run the test case with single task with period
5ms with different dutycycle, the total running time is 10s. Based on
the tracing log, we do statistics for all CPUs for all idle states
duration, the unit is second (s), on Hikey board the result shows the C2
state (the CPU deepest state) selection improvement.
Some notations are used in the table:
state: C0: WFI; C1: CPU OFF; C2: Cluster OFF
All testing cases have single task with 5ms period:
Without patches With patches
Difference
----------------------------- -----------------------------
-------------------------------
Duty cycle C0 C1 C2 C0 C1 C2
C0 C1 C2
1% 0.218589 4.208460 87.995606 0.119723 0.847116 91.940569
-0.098866 -3.361344 +3.944963
3% 0.801521 5.031361 86.444753 0.147346 0.820276 91.761191
-0.654175 -4.211085 +5.316438
5% 0.590236 2.733048 88.284541 0.149237 1.042383 90.490482
-0.440999 -1.690665 +2.205941
10% 0.601922 6.282368 84.899870 0.169491 1.304985 89.725754
-0.432431 -4.977383 +4.825884
20% 1.381870 8.531687 80.627691 0.307390 3.302562 86.686887
-1.074480 -5.229125 +6.059196
30% 1.785221 6.974483 81.083312 0.548050 5.319929 83.551747
-1.237171 -1.654554 +2.468435
40% 1.403247 6.474203 80.577176 0.467686 6.366482 81.983384
-0.935561 -0.107721 +1.406208
Leo Yan (2):
cpuidle: menu: Correct the criteria for stopping tick
cpuidle: menu: Dismiss tick impaction on correction factors
drivers/cpuidle/governors/menu.c | 55 ++++++++++++++++++++++++++++++++--------
1 file changed, 45 insertions(+), 10 deletions(-)
--
2.7.4
